The present invention is related to an ECM system adapted for jamming threat radars, in general, and more particularly, to a radar jammer which utilizes an electronically agile, sparsely populated, phase controlled antenna array of pseudo-randomly spaced radiating elements to form a high gain, single narrow beam of radiation directed at a detected threat radar, but containing only a small fraction of the available tansmitting power, while providing simultaneously therewith effective jamming radiation over a wide coverage region.
Some ECM radar jammers, particularly those adapted for use on airborne vehicles, like missiles or airplanes, for example, have severe equipment weight restrictions imposed on the power supplies thereof. Consequently, the available jamming power for transmission is adversely limited and as a result, must be used most effectively. In most cases, airborne radar jammers are required to cover large threat volumes of space which may be on the order of .+-.60.degree. azimuth and 30.degree. elevation. The range of coverage in connection with those volumes is usually dependent on the power generation of the jammer. In order to defeat most of the threat radars encountered in a coverage region, large values of effective radiated power (ERP) are required. This effective radiated power is commonly derived as the product of the transmitted power and antenna gain. Since, for airborne radar jammers, for example, the available transmission power is limited, the required levels of ERP cannot be achieved simultaneously over the entire coverage region in most cases.
If only a single threat is encountered at any time in the threat volume, the ERP of the radar jammer may be increased effectively by using a high gain antenna for forming and aiming a relatively narrow beam directly at the threat. However, it is not prudent in most scenarios to transmit all jamming power into narrow volumes of the coverage region where known threats are detected mainly because this action leaves the aircraft vulnerable to threats which may reside in other portions of the coverage region.
In a multiple threat environment, some ECM systems have proposed to sequentially aim a narrow beam at each threat, or form multiple narrow beams with the available transmitting power beam shared therebetween or distribute equally the available transmitting power over the entire coverage region. In the first case, the need for rapidly re-aiming the formed narrow beam usually requires great agility of an electronically controlled phased array and little simultaneous defeat coverage is anticipated. The second case requires a very complex electronically controlled phased array which has the capability to alter radiating element amplitudes as well as phase, however, this case suffers from a reduction of ERP in each formed beam by a factor equal to or greater than the multiplicity of beams simultaneously formed. In the third case, no enhancement of the ERP by the antenna gain is expected. All of these proposed schemes thus far represent less than optimum utilization of the available jamming power because only a very small amount of transmitted energy illuminates the threat radar antenna detected in the coverage region with the remaining transmitted energy being lost to empty space.
Typically, airborne radar jammers are designed to form a relatively narrow jamming beam of radiation with an electronically phase controlled uniformly spaced array of radiating elements. A block diagram schematic of a typical embodiment for application as an airborne radar jammer is shown in FIG. 1. Generally, an array of antenna elements . . . , a1, a2, . . . , a5, . . . is disposed on a planar antenna section 10 with a uniform spacing s between each element of the array. Coupled to each antenna element may be a conventional phase shifter denoted by the blocks labeled P.S. The radiation power may be developed in a jamming transmitter 12 and provided to a conventional power divider unit 14 which distributes the available radiating power to the individual phase shifters. In addition, a phase shift controller 16 may provide a signal to each phase shifter of the antenna array to govern the phase shifting operation occurring therein.
In operation, power developed in the jammer transmitter 12 is passed along to the power divider 14 wherein it is distributed to the antenna elements a1, a2, . . . a5 via corresponding phase shifters P.S. The phase shift controller 16 may govern the phase shifters to form and direct a beam of radiation in a direction 20 preferably towards a detected radar threat for jamming purposes by providing a phase shift signal PS1-PS5 to each phase shift circuit individually to cause corresponding delays d1, d2, . . . , d5 in the radiated energy produced by the radiating elements coupled thereto. A phase front, denoted by the dashed line 22, is rendered by the phase controlled energy radiation pattern, which in turn forms a narrow beam 24 perpendicular to the uniform phase contour 22 and in the preferred direction 20. With the uniform spacing s between the radiating elements, the width and strength of the formed beam 24 are primarily dependent on the spacing and number of radiating elements, respectively, in the antenna array 10.
The graph depicted in FIG. 2 illustrates a typical radiation pattern along the azimuth angle of the antenna resulting from uniformly spaced radiating elements. The isotropic power level, denoted by the dashed line at 0 dB power, represents the uniform radiation pattern of the antenna (i.e. without formation of a narrow beam). In the example illustrated by the graph of FIG. 2, a radiating beam is formed in the azimuth angular direction of approximately 23.degree. with a peak power level with respect to the isotropic level of approximately 15 dB. Note that the power radiation spectrum outside of the main beam pattern has been reduced substantially in most cases on the order of 5 to 7 dB's from that of the isotropic level. Undesirably then, there appears to be not much energy distributed anywhere but in the main beam pattern formed by the uniformly spaced radiating element array. Evidently, any energy not intercepted by the threat radar in the formed beam may be considered as wasted energy.
It has been proposed that if the beam width could be made narrower, not as much energy would be required and the threat radar could still be satisfactorily defeated assuming directive accuracy. Theoretically, to accomplish the results of a narrower beam all that need be done is to increase the uniform spacings between the radiating elements. However, as the spacings of the radiating elements are made greater, additional beams will also be formed around the desired beam 24 as illustrated by the dashed line patterns 25, 26, 27 and 28 in the schematic of FIG. 1. Of course, the energy radiated in the auxiliary beams is considered without purpose and as a result wasted. Accordingly, the extent of wasted energy in this case may be as much energy as there are in the other undesirably formed radiated beams. Thus, by making the spacing between the radiating elements of the antenna array broader, a narrower beam is achieved with less energy content, but in reality nothing appears to have been gained because what energy was saved by narrowing the beam size apparently is going off into spuriously formed beams with no purpose.